refactor: finish the AgentHub rename and drop the MCP server

Two leftovers from the rebrand, done together because they touch the same
docs and manifests.

**Data directory.** `agent-dashboard` could not simply be renamed in the
source: it names a real directory holding the live SQLite database. Startup
now moves it once, via `migrateLegacyDataDir()` in `server/lib/claude-home.js`
— a same-filesystem rename, so it is instant even at multiple gigabytes and
cannot leave two divergent copies. `getDataDir()` stays PURE and resolves to
the legacy path until the move happens; the first version of this put the
rename inside the getter, which meant `npm run test:server` relocated the
developer's real database as a side effect. The call sits behind
`require.main === module` and ahead of the router requires, because those pull
in db.js, which opens SQLite at module load. A failed rename is silent and the
old directory keeps being used: an unfashionable directory name beats a blank
database beside an unreachable one. Same treatment for the port-discovery file
(`.agent-dashboard.json` → `.agenthub.json`, old one dropped on write).

Not renamed on purpose: the Sagi localStorage keys (renaming silently resets
everyone's prefs) and the PWA manifest `id` (changing it orphans an installed
app).

**MCP server.** 4 tool calls in 46 days across 577,132 events, against 4,556
lines of TypeScript, a 2 MB committed build artifact, and a pre-commit guard
that had already blocked commits twice. Deleted: `mcp/`, the maintainer skills
and `mcp-reviewer` agent, the build check, `plugins/agenthub/.mcp.json`,
`docs/MCP.md`, 12 npm scripts, 5 Makefile targets, and every doc section
describing it.

Kept, because they are unrelated things sharing the name: the Config
Explorer's MCP tab and its API, per-lane Playwright MCP, the agenthub-config
plugin's MCP audit, and the historical `mcp__plugin_agenthub_*` events that
analytics must keep attributing.

Verified: server 1432/1432, client 459/459, tsc clean, prettier clean. Both
directions of the migration proven — a sandbox-HOME test run leaves the legacy
directory untouched, and a real boot moves it with its contents intact.

server/lib/server-info.js

@@ -37,6 +37,11 @@ const DEFAULT_PORT = 4820;
 
 /** Absolute path of the discovery file. */
 function getServerInfoPath() {
+  return path.join(getClaudeHome(), ".agenthub.json");
+}
+
+/** Where the discovery file lived before the AgentHub rebrand. */
+function getLegacyServerInfoPath() {
   return path.join(getClaudeHome(), ".agent-dashboard.json");
 }
 
@@ -168,6 +173,10 @@ function writeServerInfo(port) {
   try {
     const dir = getClaudeHome();
     fs.mkdirSync(dir, { recursive: true });
+    // Retire the pre-rebrand discovery file. It is dropped rather than merged:
+    // its contents are live pids and ports, all of which this write is about to
+    // re-derive, and leaving it would let an older reader resolve a dead port.
+    fs.rmSync(getLegacyServerInfoPath(), { force: true });
     const existing = readInfoFile().filter(
       (s) => Number.isInteger(s.port) && s.port > 0 && s.pid !== process.pid && isPidAlive(s.pid)
     );